ACID AND BASE STRENGTH - CHEMISTRY 102

Chemistry 102

______________________________________________________________________________________

EXPERIMENT 5

ACID AND BASE STRENGTH

PURPOSE:

1. To distinguish between acids, bases and neutral substances, by observing their effect

on some common indicators.

1. To distinguish between strong and weak acids and bases, by conductivity testing.

3. To identify an unknown, as an acid (strong or weak), a base (strong or weak) or a

neutral substance.

PRINCIPLES:

We frequently encounter acids and bases in our daily life. Acids were first associated

with the sour taste of citrus fruits. In fact, the word acid comes from the Latin word

acidus, which means, ¡°sour¡±. Vinegar tastes sour because it is a dilute solution (about 5

percent) of acetic acid; citric acid is responsible for the sour taste of a lemon. The sour

tastes of rhubarb and spinach come from small amounts of oxalic acid they contain. A

normal diet provides mostly acid-producing foods. Hydrochloric acid is the acid in the in

the gastric fluid in your stomach, where it is secreted at a strength of about 5 percent.

Water solutions of acids are called acidic solutions.

Bases have usually a bitter taste and a slippery feel, like wet soap. The bitter taste of

tonic water comes from natural base, quinine. Common medicinal antacides (used to

relieve heartburn) and bitter tasting Milk of Magnesia, a common laxative, (a suspension

of about 8 percent of magnesium hydroxide) are also bases. Other bases used around the

house are cleaning agents, such as ammonia, and products used to unclog drains, such as

Draino. The most important of the strong bases is sodium hydroxide, a solid whose

aqueous solutions are used in the manufacture of glass and soap.

Water solutions of bases are called alkaline solutions or basic solutions.

Substances used to determine whether a solution is acidic or basic are known as

indicators. Indicators are organic compounds that change color in a specific way,

depending on the acidic or basic nature of the solution. A wide variety of indicators are

commonly used in the chemistry laboratory, to identify the acidic or basic nature of an

aqueous solution. This experiment uses only two types of indicators: litmus, a vegetable

dye, and phenolphthalein.

In summary, some of the characteristic properties commonly associated with acids and

bases in aqueous solutions are the following:

ACIDS

BASES

Sour taste

Bitter taste

Change the color of litmus paper

Change the color of litmus paper

In a specific way

In a specific way

Do not change the color of phenolphthalein Change the color of phenolphthalein

React with carbonates to produce CO2

React with acids

React with bases

React with acids

Fall 2015 /

1

Chemistry 102

______________________________________________________________________________________

EXPERIMENT 5

ACID AND BASE STRENGTH

When acids and bases react with one another in equal proportions, the result is a

neutralization reaction, which produces neutral products: salt and water. Neutral means

in this context, that these products do not change the color of litmus or phenolphthalein,

do not have a sour or bitter taste, therefore they are ¡°neither acidic nor basic¡±.

The following equations represents a typical acid-base neutralization reaction:

HCl(aq)

Acid

+

NaOH(aq)

Base

Neutralization

©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤>

NaCl(aq)

Salt

+

H2O(l)

water

Note that a salt is any compound of a cation (other than H+) with an anion (other than

OH©¤ or O2©¤). The word salt in everyday conversation means sodium chloride, which is a

salt under this definition.

It appears that acid properties are often opposite to base properties, and vice versa; a base

is an anti-acid, and an acid is an anti-base.

Several theories have been proposed to answer the question ¡°What is an Acid or a Base?¡±

One of the earliest and most significant of these theories was proposed by a Swedish

scientist, Svante Arrhenius in 1884.

According to Arrhenius:

AN ACID

A BASE

Is a hydrogen-containing

substance that dissociates to

produce hydrogen ions, H+,

in aqueous solutions

Is a hydroxide-containing

substance that dissociates to

produce hydroxide ions, OH?,

in aqueous solutions.

The hydrogen ions, H+, are

produced by the dissociation

of acids in water

The hydroxide ions, OH?, are

produced by the dissociation

of bases in water

HA ????? H+ + A?

Acid

MOH ????? M+ + OH?

Base

An ACID SOLUTION contains an

excess of hydrogen ions, H+.

A BASE SOLUTION contains an

excess of hydroxide ions, OH? .

Examples: HCl(aq),

HC2H3O2(aq)

Examples:

Fall 2015 /

2

NaOH(aq),

NH4OH(aq)

Chemistry 102

______________________________________________________________________________________

EXPERIMENT 5

ACID AND BASE STRENGTH

Today we know that H+ ions cannot exist in water, because a H+ ion is a bare proton, and

a charge of +1 is too concentrated for such a tiny particle. Because of this, any H+ ion in

water immediately combines with a H2O molecule to form a hydrated hydrogen ion,

H3O+ [that is, H(H2O) +], commonly called a hydronium ion.

H+(aq)

+

H2O(l)

©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤>

H3O+(aq)

hydrogen ion

water

hydronium ion

(proton)

While it is a known fact that that the hydrogen ion does not exist alone, as H+ , but is

stable in aqueous solution in the form of the hydronium ion, H3O+, it is an accepted

simplification to represent the hydronium ion, H3O+, as a hydrogen ion, H+.

In beginning courses, formulas for acids (and no other compounds except water) are

written with the dissociable hydrogen atoms (acidic hydrogen atoms) first, as in:

HCl (aq)

Hydrochloric acid

HC2H3O2(aq) Acetic acid

Note: Only the hydrogen written first is capable of being released as

hydrogen ion, H+; the other three hydrogen atoms do not yield

H+ ions in aqueous solution

Methane

Ammonia

Urea

Glucose

CH4

NH3

NH2©¤CO©¤NH2

C6H12O6

Are not acids, since do not provide H+

ions to aqueous solutions.

Their hydrogen atoms are therefore not

written first in their formulas.

With a slight modification (the introduction of the H3O+ ion) , the Arrhenius definitions

of acid and base are still valid today, as long as we are talking about aqueous solutions.

In summary, according to Arrhenius:

When we dissolve an acid (a molecular

When we dissolve a base (an ionic

substance) in water, the molecules of acid react substance) in water, the metallic ion and

with water to produce H3O+ ions

the hydroxide ions, OH- separate

H2O

H2O

+

©¤

HCl(g) ©¤©¤©¤©¤©¤©¤©¤> H3O (aq) + Cl (aq)

NaOH(s) ©¤©¤©¤©¤©¤©¤?>Na+(aq) + OH?(aq)

Accepted simplification:

Accepted simplification:

+

©¤

HCl(aq) ©¤©¤©¤©¤©¤©¤©¤> H (aq) + Cl (aq)

NaOH(aq) ©¤©¤©¤©¤©¤©¤> Na+(aq) + OH?(aq)

The aqueous solution of hydrochloric acid

contains ions only (no molecules)

The acidic solution is a strong electrolyte.

(Complete dissociation took place).

Acids which are completely dissociated in ions

in aqueous solutions are called strong acids.

Fall 2015 /

3

The aqueous solution of sodium hydroxide

contains ions only (no molecules)

The basic solution is a strong electrolyte.

(Complete dissociation took place)

Soluble metallic hydroxides, completely

separated in aqueous solution are called

strong bases.

Chemistry 102

______________________________________________________________________________________

EXPERIMENT 5

ACID AND BASE STRENGTH

Other substances, although they do in fact produce hydrogen ions, H+, when dissolved in

water, dissociate only partially. Such substances are called weak acids. It follows that

weak acids are weak electrolytes.

For example, acetic acid, HC2H3O2, found in vinegar, is a weak acid.

partial dissociation

HC2H3O2 (aq)

< ©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤

H+(aq) +

C2H3O2 - (aq)

acetic acid

©¤©¤>

hydrogen ion

acetate ion

The double arrows in the equation for the partial dissociation of acetic acid indicate that

the dissociation reaction for this substance reaches equilibrium.

At equilibrium, a certain fixed concentration of hydrogen ion, H+, is present. The

equilibrium lies well to the left (as suggested by the size of the respective arrows) and

only a few acetic acid molecules (HC2H3O2) are converted to hydrogen ions (H +) and

acetate ions (C2H3O2 -). In a 0.1 M solution of acetic acid in water, only about 1 molecule

molecules of HC2H3O2 out of every 100 have reacted to form hydrogen ions (H+ ) and

acetate ions (C2H3O2 -).

The concentration of hydrogen ion, H+, produced by dissolving a given amount of

weak acid is much less than if the same amount of strong acid is dissolved.

A similar situation exists with bases. Some substances, which do not contain hydroxide

ions, OH? in pure form, produce hydroxide ions (OH©¤ ) in water by reacting with the

water. The most important example of this kind of base is ammonia gas, NH3.

Ammonia produces OH©¤ ions by taking H+ ions from water molecules and leaving OH©¤

ions behind:

H+

NH3(g)

ammonia

+

H2O(l)

water

partial dissociation

< ©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤©¤ NH4+(aq)

+

OH©¤ (aq)

©¤©¤>

ammonium ion

hydroxide ion

(ammonium hydroxide)

This equilibrium also lies well to the left, meaning that the majority of particles present

in an aqueous solution of ammonia are NH3 molecules and very few ammonium ions,

(NH4+) and hydroxide ions (OH©¤) are present. In a 1 M solution of NH3 in water, only

about 4 molecules of NH3 out of every 1000 have reacted to form NH4+ ions.

Nevertheless, some OH©¤ ions are produced, so an aqueous solution of NH3 is in fact a

base, although a weak one.

Substances that produce OH? by partial dissociation are called weak bases. It

follows that weak bases are weak electrolytes.

As with weak acids, the concentration of hydroxide ions (OH©¤) in a solution of a weak

base is much smaller than if the same amount of strong base had been dissolved.

Fall 2015 /

4

Chemistry 102

______________________________________________________________________________________

EXPERIMENT 5

ACID AND BASE STRENGTH

Although the Arrhenius definitions of acids and bases have proved very useful, the theory

is restricted to the situation of aqueous solutions.

In 1923 new definitions of acids and bases were proposed simultaneously by Bronsted

and Lowry. The Bronsted/Lowry theory of acids and bases extends the Arrhenius

definitions to more general situations, which explain the behavior of weak bases and do

not require the solvent to be water.

According to the Bronsted/Lowry theory:

AN ACID:

A BASE:

+

Is a proton, H , donor

Is a proton, H+, acceptor

AN ACID-BASE REACTION (NEUTRALIZATION REACTION) IS THE

TRANSFER OF A H+

H+

ACID

+

BASE

??????

SALT

+

WATER

H+

+ NaOH(aq) ??????

HCl(aq)

NaCl(aq) +

H2O(l)

In summary, both acids and bases have characteristic properties and can either be strong

or weak, as shown below:

ACIDS

BASES

Arrhenius

definition

Bronstead/Lowry

definition

Electrolyte

Strength

Extent of

dissociation

produce H+ ions in

aqueous solution

H+ donors

produce OH? ions in

aqueous solution

H+ acceptors

STRONG

ACIDS

(strong

electrolytes)

completely

dissociated

WEAK

ACIDS

(weak

electrolytes)

partially

dissociated

STRONG

BASES

(strong

electrolytes)

completely

dissociated

WEAK

BASES

(weak

electrolytes)

partially

dissociated

ions only

mostly

molecules and a

few ions

ions only

mostly

molecules and a

few ions

Symbols used to

show extent of

dissociation

Particles present

solution

Fall 2015 /

5

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download